Composite material and use of a composite material

ABSTRACT

Composite material with a polymerisable organic binder, characterised in that it contains a filler with filler particles which have the shape of a torus.

BACKGROUND OF THE INVENTION

Composite materials are composites made from a plastics material andinorganic filling materials. Conventionally, therefore, they consistprimarily of three different components: a polymerisable organic matrix,filler particles and an agent which ensures the bond between the polymerand the filler particles. Dental restorative materials represent aspecific form of composite materials as they are subjected to thegreatest demands, due to the extreme physical and chemical stress in theextremely inhospitable environment of the mouth. Due to their extensiverequirement profile, these materials often serve as a basis fordeveloping non-dental composites or as a model for use in the non-dentalfield.

Dental restorative composite materials have been used for over 40 yearsfor fillings, linings and fixings, as stump restoration, crown andbridge, prosthesis and relining material, as filled adhesives whichproduce adhesion on dental enamel, plastics, ceramics or metal, and asdental sealing compositions. After being introduced into the cavity,composites cure in a polymerisation reaction, either chemically or bythe addition of external energy.

The organic polymerisable component of the dental composite material isgenerally cross-linked in a radical reaction and contains correspondingethylenically unsaturated functional groups. The monomers and oligomerscomprise mono-, di- and/or polyacrylates and/or methacrylates, such asfor example diglycidylmethacrylate of bisphenol A (‘Bis-GMA’,2,2-bis[4(2-hydroxy-3-methacryloxypropyloxy)-phenyl]propane) anddiurethane di(meth)acrylate from2,2,4-Trimethylhexamethylenediisocyanate and2-hydroxyethyl(meth)acrylate (UDMA). When referring to methacrylates,analogous acrylates are also understood. Commercially obtainablestandard mixtures contain Bis-GMA, UDMA andtriethyleneglycoldimethacrylate to reduce the viscosity.

In order to be able to carry out radical curing of the resincomposition, an initiator system is added to the mass which triggers theradical polymerisation following radiation and/or a redox reactionprocess. A typical system which initiates the radical polymerisation ofthe methacrylate consists of a photoinitiator (ketone) and anaccelerator (amine). Typically, camphorquinone is used as a ketone andpara N,N-Dimethylaminobenzoic acid as an amine. Further photoactivecomponents can be admixed to the mixture. If the composition is exposedto light with a suitable radiation source at 460 nm, the compositematerial is photochemically cross-linked. Alternatively, the materialcan also be chemically cross-linked. To this end, the peroxide/tertiaryamine combination is used as a redox system. The two components have tobe kept separated from one another in a 2-component system. After mixingthe two components free radicals are generated and the radicalpolymerisation of the acrylates cures the composite material. As noexternal aids are required with this type of curing, this system is alsoknown as self-curing.

Composite compositions can therefore be designed as either self-curingor photochemically curing (mono-cure). Furthermore, compositecompositions can be formulated which represent a combination ofself-curing and photochemically curing systems (dual-cure). Ifpolyacrylic acid or a derivative of polyacrylic acid is added to onepart of the ‘dual-cure’ composite system and basic glass is present inthe other part, under suitable conditions, this system also cures in anacid base reaction (triple-cure) in addition to a chemical andphotochemical mechanism.

The inorganic filling materials of the dental composite materialgenerally consist of quartz, borosilicate glass, lithium aluminiumsilicate, barium aluminium silicate, strontium/barium glass, zinc glass,zirconium silicate, pyrogenic or colloidal silicic acid.

The bond of the inorganic filling materials with the organic resinmatrix is generally ensured by the use of coupling agents or adhesionpromoters. This is essential for the subsequent suitability of thecomposite mass as a dental material. In this connection, the filler,generally in the presence of weak acids, is treated with a silane beforeit is mixed with the liquid resin component. The method for preparingsilanized filler surfaces consists in firstly adjusting an ethanol/watermixture (generally 95/5% by volume) with acetic acid to a pH value of4.5-5.5. The silane is then added in such an amount that a solutionstrength of ca. 2% results. Within 5 minutes the alkoxysilyl groups arehydrolysed and the siloxane formation commences. Then the filler to beprocessed is added to the solution by continuous mixing. Within a fewminutes the silane is adsorbed by the filler and the surface of thefilling material loaded with adhesion promotor. The solution is decantedoff and the particles washed twice with ethanol. Finally, the remainingsilanol functions are condensed for a few minutes at 110 DEG C and 24hours at room temperature.

The silane acts as a surface active material which compatabilises thesurface of the filler with the resin matrix and ensures a rigid bondbetween the organic and inorganic material. Amongst others,3-methacryloyloxypropyltrimethoxysilane has proved to be a particularlysuitable silane for creating a bond between the inorganic and organicphase. A portion of the hydrolysed alkoxysilyl groups of the silanereacts directly with the hydroxyl groups on the mineral surface of thefiller, while the other portion fuses together and thus produces acontinuous layer of the coupling agent on the filler surface. During thecourse of the subsequent radical polymerisation of the dental compositemass, the methacryloyloxypropyl functions of the silane layercontinuously adhering to the filler surface are then polymerised in theorganic resin phase and thus form a permanent bond between thehydrophilic fillers and the hydrophobic resin matrix.

The properties of the resulting dental composite material is determinedprimarily by the inorganic phase. Whilst Young's modulus (E-module) foran unfilled resin system based on Bis-GMA is 2.8 GPa, the dental enamelhas a value of 83 GPa and the dentine a value of 19 GPa. By adding aconventional silylated filler to the Bis-GMA-resin the value of 2.8 GPacan be markedly improved. If the filler is added to the resin in thevolume ratio of 1 to 1.25, Young's modulus can be raised to a value of10 GPa. For a ratio of 1:1 a value of 15 GPa can be achieved.

The type of filler, the amount and distribution of particles for a givenresin composition determine the mechanical, aesthetic and rheologicalcharacteristics of the dental composite moulded material, such assurface hardness, abrasion resistance, wear resistance, pressureresistance, tensile strength, polymerisation shrinkage, fractureresistance and thermal shock resistance as well as polishability, shine,opacity, translucence and colour stability, as well as flowcharacteristics, stability and modelability. As a rule of thumb, thehigher the concentration of silanized filler in the liquid resin, thebetter the mechanical, physical and chemical properties of the curedmoulded material.

Against the background of the paramount importance of the inorganicphase for the properties of dental composite materials, the traditionaldivision of dental composite materials is understood to be into threedifferent basic groups.

A macrofilled composite material is a highly filled composition (up to87 wt. %) with relatively large particles (1-100 μm). Whilst previously,glass powder with average particle sizes of 30-50 μm served as thefiller, nowadays the filler is generally ground quartz or even glassceramics with an average particle size of 8-12 μm. Macrofilledcomposites have the best wear resistance, but due to the particle sizehigh polishing is extremely difficult. During polishing the bulky fillerparticles break out of the filling, small holes remain behind and thebroken-out filler fragments exert an abrasive effect on the remainingmoulded material, so that macrofilled composites cannot be highlypolished and have a fundamental aesthetic flaw.

In order to comply with the demand for improved aesthetics, the group ofmicrofilled dental composite materials was developed. A characteristicfeature of these groups is the exceptionally small particle size of thecomposite filler which primarily consists of amorphous silicic acid andhas an average particle size of ca. 0.04 μm. This small particle sizeresults in an extremely large particle surface which, due to intensiveinteraction forces between the particle surfaces, in turn sets apremature limit for the filler concentration of the composite material.As a rule, microfilled composite materials cannot be mixed with fillerof more than 50 wt. %, as the material is then no longer workable due tohigher viscosity. This composite group may be highly polished, exhibitsexcellent refractive properties and fulfils all criteria of anexceptionally aesthetically effective dental material. As a result ofthe low filler content, microfilled materials, compared to macrofilleddental composites, however, exhibit considerably reduced mechanicalproperties such as abrasion, tensile strength, excessive shrinkage, etc.

It has been attempted on many occasions, but until now without success,to increase the filler content, for example by the incorporation ofpyrogenic silicic acid into prepolymerised resin particles (25 μm),agglomerated or sintered particles and thus to increase the strength.

By attempting to combine the polishability of the microfilled compositeswith the excellent mechanical properties of the macrofilled composites,the group of so- called hybrid composites was developed. In thisconnection, the filler used is a mixture of conventional glass with aparticle size of 0.6-1.5 μm and of nanoscale particles of 0.01-0.05 μm.As a rule, the quantifiable portion of nanoscale silicic acid particlesis 7-15 wt. %. The entire filler content can be up to 80 wt. %. Due tolarge variations in the particle sizes, an exceptionally compact packingdensity of the filler particles can be achieved, smaller particles beinglocated in the spaces between the larger particles.

An example for the composition of a microfilled system is disclosed inDE 2403211. Hybrid materials are known from the patents DE 2405578, DE3403040 and EP 382033.

OBJECT OF THE INVENTION

Although, due to improvements in materials science, modem compositefillers are a permanent fixture in the treatments available to dentists,even in the side tooth area, these systems nevertheless have severalfundamental drawbacks which are primarily linked with the ‘bond' betweenthe organic resin matrix and the inorganic filler surfaces. The silanecoupling agents form ‘siloxane bonds’ with minerals. These bonds whichensure the bond between the two phases, may be hydrolysed, like any bondbetween an organic polymer and a hydrophilic, mineral material surface.Hydrolysis of the siloxane bond however produces hydrolytic degradationin the polymer, increased crack formation along the material/resininterfacial region, water absorption, softening effects in the polymer,swelling of the composite, reduced wear resistance, abrasion resistanceand colour stability, due to the filler breaking out. Finally, the bondof the two phases is broken.

The advantage of silane relative to other adhesion promoters lies in itscharacteristic of behaving in a reversible manner with regard tohydrolytic bond cleavage. The thermodynamic equilibrium lies broadly onthe side of the siloxane bond formation. Although the equilibrium amountof water molecules is therefore more important on the polymer/solidinterface layer than the diffusion rate of the water in the polymer,water entering the material will however set the hydrolytic degradationprocess in motion. In the presence of strongly hydrophobic resins, wateritself reaches the polymer/solid interface by diffusion. Once theinterface layer is attacked, the water is attached there in the form ofclusters, the bond of the organic phase to the inorganic phase isloosened and the structure of the composite broken up by osmoticpressure.

To improve the bond between filler and polymer matrix, the possibilitywas considered to create a physical adhesion in addition to the chemicaladhesion. In U.S. Pat. No. 4,215,033 a semi-porous filler is produced byetching glass. Microporous fillers for use as dental materials are knownfrom the publications U.S. Pat. No. 4,217,264, EP 4868, EP 172513, DE19846556 and DE 19615763. With the physical adhesion, resin penetratesinto the pores of the filler and thus after polymerisation anchors theorganic with the inorganic phase, as the cured resin is held tightly inthe pores of the filler. Thus an improved structural integrity of themoulded material is ensured.

The principle of the physical anchoring of filler and matrix which isdisclosed in DE 19615763 and includes the use of porous SiO₂ particles,has however three principal disadvantages. The first consists of theextremely expensive production of the porous filling materials whichincludes a very expensive phase separation step, and grinding andscreening processes. The second disadvantage lies in the very small porediameter which is preferably 90-100 nanometers. In order to ensure aneffective inflow of the resin into the pores, resin composites of verylow viscosity must be used with low surface tension. This is obtained bythe use of dimethacrylates with a low molecular weight, such as forexample triethyleneglycoldimethacrylate (TEDMA) orhexanedioldimethacrylate (HDDMA). A higher proportion of these lowmolecular monomers leads however to increased composite shrinkage.Alternatively, the viscosity of the matrix can also be reduced by theaddition of monomethacrylates, such as for examplehydroxypropylmethacrylate (HPMA) ortriethyleneglycolmonoethylethermonomethacrylate. The use ofmonomethacrylates leads to a poorer cross-linking of the polymer,compared to dimethacrylates and thus to lower flexural strength andgreater discolouration. The third disadvantage lies in the restrictionof the production process to silicon dioxide fillers which do not allowa clinically acceptable radiopacity to be set.

In spite of enormous improvements in the field of dental compositematerials, the problem of the phase bond however remains unsolved, aseven with the use of porous fillers, it can lead to the release of thepolymer matrix from the inorganic filler by means of hydrolyticcleavage. It is therefore the object of the invention to provide afiller which forms a stable bond with the organic phase and allows sucha strong physical bond between it and the binder of the dental materialthat possible hydrolysis can no longer destroy the bond once it isformed, and a composite material containing this filler which, due tothe stable bond between the phases, ensures improved properties relativeto the prior art.

DETAILED DESCRIPTION OF THE INVENTION

The object is achieved by a composite material according to claim 1, bya dental composite material according to claim 16 and by the use of acomposite material according to claim 17. Advantageous embodiments ofthe invention are disclosed in the sub-claims.

The invention relates to a specific filler with filler particles whichcomprise a spherical annular structure and structurally correspond tothe geometry of a torus. In contrast to porous fillers, in which theresin is merely mechanically anchored in the pores of the fillerparticles, here the extreme case is realised where the filler iscompletely drawn through by a single pore. This leads to the organicphase being continually bonded with the inorganic phase and thus forms aparticularly effective linkage from a mechanical point of view. In thisconnection, the torus-shaped filler particles are mechanicallypenetrated by binders, like a string of beads, and bonded to one anotherby the continuous resin phase present in the interior of the torus, suchthat they can no longer be released from the resin matrix by hydrolyticdegradation. Thus a dental material is produced which, due to theextremely effective bond between the organic and inorganic compositephase, has a particularly pronounced abrasion resistance with highflexural strength at the same time, which cannot be achieved bycomparable dental materials of the prior art. As the hydrolyticdegradation which starts over a period of time, also no longer leads toa phase separation because of the solid phase bond, the durability ofthe dental material is extended with full functionality. At the sametime the aesthetic character of the dental mass is also increased.During polishing the filler particles, due to the solid phase bond withthe binder, are worn away in layers and not, as with macrofilledcomposites, broken as a whole from the polymer matrix. This allows ahigh polish.

Suitable binders for composite materials are, in addition toethylenically unsaturated monomers and oligomers, epoxides, ormocers,ceramers, liquid crystal systems, spiro-orthoesters, oxethanes,polyurethane, polyester, A-silicon and C-silicon, polycarboxylic acids,etc.

Further possible components of the composite material include colorants,pigments, stabilisers, co-initiators, wetting agents, radiopacifiers,etc.

Moreover, the invention further relates to composite materials whichinclude the filler with torus-shaped filler particles for non-dentalpurposes and to the filler with torus-shaped filler particles for anypurposes.

A further aspect of the invention relates to the following disclosedmethod for producing the torus-shaped filler particles.

The annular, spherical fillers can be created from amorphous, nanoscaleSiO₂ primary particles. To this end, colloidal silica gels arepreferably used as suspension in water (silica sols). These silica gelscan contain both ammonium, aluminium and sodium as stabilisingcounterion. The preferred primary particle size is approx. 5-100 nm, thegenerally preferred primary particle size being 10-50 nm. Common SiO₂suspensions are, for example, Ludox AS40 or Ludox HS40 (Aldrich ChemicalCompany, Milwaukee, USA).

In order to provide sufficient radio-opacity of the dental material,heavy metal oxides in combination with SiO₂ are incorporated in theannular fillers. Preferably the heavy metal oxides are used with anatomic number of greater than 28. Oxides of yttrium, strontium, barium,zirconium, tungsten, tin, zinc, lanthanum or ytterbium or combinationsthereof are particularly preferably used. The heavy metals can beincorporated in the production process in the form of solutions, sols orparticle suspensions. In this connection, the preferred size of theheavy metal particle is 5-100nm, the particularly preferred size, 10-50nm. As precursors for the heavy metal oxides, water soluble inorganic ororganic salts of the corresponding metals, such as for example salts ofaliphatic mono or dicarboxylic acids or even alkoxides can be used.Preferably zirconium acetate is used. The element ratio silicon: heavymetal can therefore be 0.3:1 to 20:1. Preferably the ratio is 2:1 to8:1.

To produce the torus fillers, either the pure silica sols and/or aqueousmixtures of silica sols and the heavy metal salts were liberated fromwater and other volatile components. The preferred method to producespherical, micron-sized non-agglomerated particles is represented byspray drying such sols. To this end, a spray dryer ‘Mobile Minor 2000’from Niro A/S, Soborg, Denmark was used. Different geometries of nozzlewere tested (two-fluid, centrifugal and fountain nozzles), with incomingair temperatures in the region of 150-300 DEG C, material concentrationsin the region of 140 wt. %, spray pressure in the region of 2-5 bar andflow rates in the region of 0.2-2.0 Kg/h. Surprisingly, it was foundthat in the upper region of the different process parameters, particleswere formed with a torus shape. FIG. 1 shows SEM pictures of such SiO₂particles which have a torus structure.

Filler particles with an external diameter in the region of 0.5-100 μm,preferably in the region of 1-50μm can be produced. The diameter of thering openings lies in the region of 0.2-20 μm, preferably in the regionof 0.4-4.0 μm.

The particles obtained by spray drying showed moderate mechanicalstability and could be destroyed by the effect of shear forces whichoccur during the production of highly viscous dental filling material. Ahigh mechanical strength of the particles is achieved by calcination.Post-curing of the fillers was carried out at 400-1200 DEG C, preferablyat 600-900 DEG C. After post-curing, no reduction in size of theparticles is observed. The torus structure also remains unchanged afterthe thermal condensation (FIG. 2). Post-curing at higher temperatures(800-900 DEG C) leads to filler agglomerates (FIG. 3) which, however,can again be deagglomerated by adding mechanical energy, such as forexample ultrasound, and by obtaining the annular structure. The fillersaccording to the invention are silanized, in order to allow bothadditional chemical linkage of the fillers to the matrix after curingthe composite, and hydrophobing the particle surfaces, which facilitatesthe flow-through of the filler with the hydrophobic matrix. To this end,100 g of the filler is added to a solution of 5 gmethacryloyloxypropyltrimethoxysilane and 5 g water in 90 g ethanolwhich is adjusted with acetic acid to a pH value of 5, the solutiondecanted off after mixing, the filler washed with ethanol, isolated andsubsequently dried for 10 minutes at 110 DEG C and 24 hours at roomtemperature.

EMBODIMENTS

The characteristics of the torus fillers are examined with reference toexperimental light curing dental filling composites. To this end, thefollowing composites are formulated in a vacuum planetary mixer anddeaerated at low pressure of 0.95 bar.

Triethyleneglycoldimethacrylate (TEDMA), diglycidylmethacrylate ofbisphenol-A (BisGMA) and diurethane dimethacrylate (UDMA) were used asethylenically unsaturated resin components. In example 5 a 40% sol ofSiO₂ particles with a primary particle size of 1 5nm was used in adimethacrylate mixture Bis-GMA:UDMA:TEDMA of 5:3:2.

In addition to the torus particles according to the invention with anaverage particle size of 3.0 μm, spherical silicon dioxide particleswere used with an average particle size of 3.0 μm, and fragment-likebarium-aluminium boron silicate glasses with an average particle size of3.0 or 0.7 μm. The glass fillers were silanized according to the sameprocess as the torus particles.

Pyrogenic silicic acids (HDK H2000, Wacker, Munich) were used to adjustthe consistency to be suitable for processing.

Camphorquinone/4-(N,N-dimethylamino) benzoic acid ethylester (DMABEE)was used as an initiator system for the blue light curing.

The storage stability of the materials was increased by the addition ofbutylated hydroxytoluene (BHT).

The curing of the sample to carry out material scientific investigationstook place with the halogen light apparatus Polofil Lux (VOCO GmbH,Cuxhaven) with a light intensity of 750 mW/cm².

To characterise the strength of the experimental filling composite theflexural breaking strength was determined in accordance with ISO 4049,pt. 2.11. The measurement of the polymerisation shrinkage was determinedby a dilatometer 30 minutes after exposure. Apparatus construction andmethods are disclosed in the publication ’Curing contraction ofcomposites and glass-ionomer cements, A. J. Feilzer, A. J. De Gee, C. L.Davidson, Journal of Prosthetic Dentistry, Vol 59, No. 3, p297-300′. Theabrasion resistance of the composite was measured by means of thethree-body-wear, disclosed in ‘Occlusal wear simulation with the ACTAwear machine, A. J. De Gee, P. Pallav, J. Dent. Suppl. 1 1994, 22,p21-27’.

Example 1

Ba—Al—Boron silicate glass, 3.0 μm 61.2 g Pyrogenic silicic acids 4.9 gBis-GMA 16.9 g UDMA 10.1 g TEDMA 6.7 g Camphorquinone 0.07 g DMABEE 0.07g BHT 0.02 g

Example 2

Spherical SiO₂-Particle, 3.0 μm 61.2 g Pyrogenic silicic acids 4.9 gBis-GMA 16.9 g UDMA 10.1 g TEDMA 6.7 g Camphorquinone 0.07 g DMABEE 0.07g BHT 0.02 g

Example 3

Torus-particle, 3.0 μm 61.2 g Pyrogenic silicic acid 4.9 g Bis-GMA 16.9g UDMA 10.1 g TEDMA 6.7 g Camphorquinone 0.07 g DMABEE 0.07 g BHT 0.02 g

Example 4

Torus-particle, 3.0 mm 56.2 g Ba—Al—Boron silicate glass, 0.7 mm 10.0 gPyrogenic silicic acid 4.9 g Bis-GMA 14.9 g UDMA 8.1 g TEDMA 5.7 gCamphorquinone 0.07 g DMABEE 0.07 g BHT 0.02 g

Example 5

Torus-particle 58.5 g 40% SiO₂-Sol 41.3 g Camphorquinone 0.07 g DMABEE0.07 g BHT 0.02 g

Flexural Strength(MPa) Abrasion(μm) Shrinkage (Vol %) Example 1 102 773.85 Example 2 104 61 3.41 Example 3 135 44 3.09 Example 4 131 39 2.84Example 5 144 29 2.49

The exchange of fragment-like glass particles (Example 1) for sphericalsilicon dioxide particles (Example 2) leads to only a slight improvementof the abrasion resistance and the polymerisation shrinkage.

The comparison with a standard composite composition (Example 1) showsthat the replacement of fragment-like glass fillers by the torus fillersaccording to the invention, with otherwise the same recipe parameters,(Example 3) leads to a greater strength due to the optimal anchoring ofthe fillers in the cured matrix. This specific anchoring also leads toclearly improved abrasion resistance as the fillers can only be brokenout of the piece with difficulty. The high space-filling properties ofthe inorganic material, due to the spherical structure and favourablepacking of the particles bonded thereto, results in lower volumeshrinkage. By the addition of fine glass fillers to the torus particles(Example 4) the inorganic filler component can once again be increased,so that the composite has improved abrasion and shrinkage values.

The use of nanoscale SiO₂ particles (Example 5) appears to lead tooptimal space filling of the filling materials which once again improvesthe mechanical properties.

1. Composite material with a polymerisable organic binder and a fillerin a quantity of 1 to 90 wt. %, characterised in that it contains fillerparticles obtained by spray drying, which have the shape of a torus andan average external diameter in the region of 0.5-100 μm.
 2. Compositematerial with a polymerisable organic binder, characterised in that itcontains a filler with filler particles, which have the shape of a torusand an average external diameter in the region of 0.50-100 μm and inthat it additionally contains a silica sol.
 3. Composite materialaccording to claim 2, characterised in that the filler particles withthe shape of a torus are obtained by spray drying.
 4. Composite materialaccording to either claim 2, characterised in that the filler contains50 to 100 wt. % of the filler particles with the shape of a torus. 5.Composite material according to claim 1, characterised in that thefiller contains additional fragment-shaped and/or spherical inorganicfiller particles.
 6. Composite material according to claim 1,characterised in that the filler additionally contains non-torus-shapedfiller particles made from silicon dioxide.
 7. Composite materialaccording to claim 6, characterised in that the non-torus-shaped fillerparticles are produced from pyrogenic and/or precipitated silicic acidand/or silicon dioxide sols and/or from a dispersion of pyrogenic and/orprecipitated silicic acid.
 8. Composite material according to claim 1,characterised in that the torus-shaped and/or non-torus-shaped fillerparticles are silanized.
 9. Composite material according to claim 1,characterised in that the organic binder includes at least one of thefollowing materials: ethylenically unsaturated monomers and oligomers,epoxides, ormocers, ceramers, liquid crystal systems, spiro-orthoesters,oxethane, polyurethane, polyester, A-silicon and C-silicon, polycarbonicacids.
 10. Composite material according to claim 1, characterised inthat the organic binder cures chemically and/or photochemically. 11.Composite material according to claim 1, characterised in that thetorus-shaped filler particles have an average external diameter in theregion of 1 and 50 μm.
 12. Composite material according to claim 1,characterised in that the torus-shaped filler particles have an internaldiameter in the region of 0.2-20 μm.
 13. Composite material according toclaim 12, characterised in that the torus-shaped filler particles havean internal diameter in the region of 0.4-4.0 μm.
 14. Composite materialaccording to any of claims 1 to 13 claim 1, characterised in that itcontains 15-70 wt. % filler with torus-shaped filler particles. 15.Composite material according to claim 1, characterised in that thefiller particles contain silicon dioxide and/or heavy metal oxides withan atomic number of greater than
 28. 16. Composite material according toclaim 15, characterised in that the heavy metal oxides are selected fromthe group of zirconium oxide, ceroxide, tin oxide, zinc oxide, yttriumoxide, strontium oxide, barium oxide, lanthanum oxide, bismuth oxide andcompounds thereof.
 17. Dental composite material according to claim 1.18. Use of a filled and polymerisable composite material which containsa filler with filler particles which have the shape of a torus, inparticular according to claim 1, as a dental material.